Add U2F migration guide

NEWS

@@ -5,6 +5,7 @@ New features:
 * New method `RegisteredCredential.builder().publicKeyEs256Raw(ByteArray)`. This
   is a mutually exclusive alternative to `.publicKeyCose(ByteArray)`, for easier
   backwards-compatibility with U2F-formatted (Raw ANSI X9.62) public keys.
+* "Migrating from U2F" section added to project README
 
 
 == Version 1.11.0 ==

README

@@ -462,6 +462,70 @@ PublicKeyCredentialCreationOptions request = rp.startRegistration(
 ----------
 
 
+== Migrating from U2F
+
+This section is only relevant for applications that have user credentials registered via the
+link:https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-javascript-api-v1.2-ps-20170411.html[U2F JavaScript API].
+New WebAuthn deployments can skip this section.
+
+The WebAuthn API is backwards-compatible with U2F authenticators,
+and credentials registered via the U2F API will continue to work with the WebAuthn API with the right settings.
+
+To migrate to using the WebAuthn API, you need to do the following:
+
+* Follow the link:#getting-started[Getting started] guide above to set up WebAuthn support in general.
++
+Note that unlike a U2F AppID, the WebAuthn link:https://developers.yubico.com/java-webauthn-server/JavaDoc/webauthn-server-core-minimal/latest/com/yubico/webauthn/data/RelyingPartyIdentity.RelyingPartyIdentityBuilder.html#id(java.lang.String)[RP ID]
+consists of only the domain name of the AppID.
+WebAuthn does not support link:https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-appid-and-facets-v1.2-ps-20170411.html[U2F Trusted Facet Lists].
+
+* Set the
+  link:https://developers.yubico.com/java-webauthn-server/JavaDoc/webauthn-server-core-minimal/latest/com/yubico/webauthn/RelyingParty.RelyingPartyBuilder.html#appId(com.yubico.webauthn.extension.appid.AppId)[`appId()`]
+  setting on your `RelyingParty` instance.
+  The argument to the `appid()` setting should be the same as you used for the `appId` argument to the
+  link:https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-javascript-api-v1.2-ps-20170411.html#high-level-javascript-api[U2F `register` and `sign` functions].
++
+This will enable the link:https://www.w3.org/TR/2021/REC-webauthn-2-20210408/#sctn-appid-extension[`appid`]
+and link:https://www.w3.org/TR/2021/REC-webauthn-2-20210408/#sctn-appid-exclude-extension[`appidExclude`]
+extensions and configure the `RelyingParty` to accept the given AppId when verifying authenticator signatures.
+
+* Generate a link:https://www.w3.org/TR/2021/REC-webauthn-2-20210408/#user-handle[user handle] for each existing user
+  and store it in their account,
+  or decide on a method for deriving one deterministically from existing user attributes.
+  For example, if your user records are assigned UUIDs, you can use that UUID as the user handle.
+  You SHOULD NOT use a plain username or e-mail address, or hash of either, as the user handle -
+  for more on this, see the link:https://www.w3.org/TR/2021/REC-webauthn-2-20210408/#sctn-user-handle-privacy[User Handle Contents]
+  privacy consideration.
+
+* When your `CredentialRepository` creates a `RegisteredCredential` for a U2F credential,
+  use the U2F key handle as the
+  link:https://developers.yubico.com/java-webauthn-server/JavaDoc/webauthn-server-core-minimal/latest/com/yubico/webauthn/RegisteredCredential.RegisteredCredentialBuilder.html#credentialId(com.yubico.webauthn.data.ByteArray)[credential ID].
+  If you store key handles base64 encoded, you should decode them using
+  link:https://developers.yubico.com/java-webauthn-server/JavaDoc/webauthn-server-core-minimal/latest/com/yubico/webauthn/data/ByteArray.html#fromBase64(java.lang.String)[`ByteArray.fromBase64`]
+  or
+  link:https://developers.yubico.com/java-webauthn-server/JavaDoc/webauthn-server-core-minimal/latest/com/yubico/webauthn/data/ByteArray.html#fromBase64Url(java.lang.String)[`ByteArray.fromBase64Url`]
+  as appropriate before passing them to the `RegisteredCredential`.
+
+* When your `CredentialRepository` creates a `RegisteredCredential` for a U2F credential,
+  use the
+  link:https://developers.yubico.com/java-webauthn-server/JavaDoc/webauthn-server-core-minimal/latest/com/yubico/webauthn/RegisteredCredential.RegisteredCredentialBuilder.html#publicKeyEs256Raw(com.yubico.webauthn.data.ByteArray)[`publicKeyEs256Raw()`]
+  method instead of link:https://developers.yubico.com/java-webauthn-server/JavaDoc/webauthn-server-core-minimal/latest/com/yubico/webauthn/RegisteredCredential.RegisteredCredentialBuilder.html#publicKeyCose(com.yubico.webauthn.data.ByteArray)[`publicKeyCose()`]
+  to set the credential public key.
+
+* Replace calls to the U2F
+  link:https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-javascript-api-v1.2-ps-20170411.html#high-level-javascript-api[`register`]
+  method with calls to `navigator.credentials.create()` as described in link:#getting-started[Getting started].
+
+* Replace calls to the U2F
+  link:https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-javascript-api-v1.2-ps-20170411.html#high-level-javascript-api[`sign`]
+  method with calls to `navigator.credentials.get()` as described in link:#getting-started[Getting started].
+
+Existing U2F credentials should now work with the WebAuthn API.
+
+Note that new credentials registered on U2F authenticators via the WebAuthn API
+are NOT backwards compatible with the U2F JavaScript API.
+
+
 == Architecture
 
 The library tries to place as few requirements on the overall application